TY - JOUR
AU - Apak,, Reşat
AB - Abstract Quercetin (QC) is one of the most prominent dietary antioxidants present in vegetables/fruits, specifically in onions that rank second in consumption following tomato. QC with proven health benefits is now largely utilized as a nutritional supplement. In this work that aims to isolate QC from red onion peels forming a huge agricultural waste, a QC-molecularly imprinted polymer (QC-MIP) in a molar ratio of 1:4:20 (QC:4-vinylpyridine:ethylene glycol dimethacrylate) was prepared thermally through bulk polymerization. Molecularly imprinted solid phase extraction (MISPE) procedures were applied for the selective pre-concentration and purification of QC from both red onion peel methanolic extract with 58% recovery and from the extract hydrolyzate with 86% recovery. The hydrolysis process increased both the QC amount as expected and the recovery yield due to changing matrix components. The results demonstrated that onion peel can easily and efficiently be converted to a valuable product, QC, using QC-MIP as SPE sorbent. Introduction Industrial processing of plant-derived raw materials generates large amounts of by-products (wastes). These wastes constitute a serious environmental problem due to their enormous amounts and microbial decay. On the other hand, they are an abundant source of valuable compounds, in particular secondary plant metabolites such as phenolic compounds and plant cell wall materials, which can be recovered and used as biologically active components or as natural food ingredients thereby replacing synthetic additives. Therefore, by-products of plant food processing have attracted intense interest from the industry and scientific community from both economic and environmental viewpoints (1). Onions (Allium cepa L.) are one of the world’s oldest cultivated versatile vegetables that are used up fresh as well as in the form of processed products in different countries and rank second in the world after tomato (2, 3). The onion production in 2017 is almost 98 million tons in the world and 10 million tons in Europe (4). Onion is one of the major sources of various biologically active phytomolecules, e.g., phenolic acids, flavonoids, organosulfur compounds, sugars and amino acids (2, 5, 6). About 37% of fresh onions are discarded during processing as waste. The onion waste includes onion skin generated peeling, two outer fleshy scales, roots, top and bottom of the bulbs and also undersized, malformed, diseased or damaged bulbs. These onion waste products are an environmental concern, because they are unsuitable for organic fertilizer or fodder due to their characteristic strong aroma and rapid development of phytopathogens. The onion processing industry is being forced to develop innovative methods to solve this problem (2, 5). One way could be the usage of onion wastes as a natural source of high-value products such as quercetin (QC) and other functional ingredients. Therefore, there are a great number of studies about characterization, phenolic composition and recovery of bioactive ingredients from onion wastes (2, 3, 7–15). Flavonols and anthocyanins (in only red and pink onion varieties) are two major groups of flavonoids in onions. QC 4′-glucoside and QC 3,4′-diglucoside are reported as the main flavonols in onions, accounting for about 80 to 95% of total flavonols (16). Onion skin—the non-edible dry peel—is richer in total flavonoids compared to the edible flesh (2, 13–18). The flavonoids present in the peel are mainly aglycones due to flavonol glucoside hydrolysis during peel formation. QC is concentrated in the dry skin of most onions where its oxidation products, 3,4-dihydroxybenzoic acid and 2,4,6-trihydroxyphenylglycosilic acid, impart the brown color and provide the onion bulb protection from soil microbial infection (16). QC (3,3′,4′,5,7-pentahydroxyflavone), one of the most well-known dietary flavonols belonging to the flavonoid class of polyphenolic compounds, has been included in human diet for a long history. Primary sources of QC are vegetables such as onions, asparagus and red leaf lettuce and fruits such as apples, cherries and berries including grape (19). Especially, onions as widely consumed vegetables ranked the highest QC content in a study of 28 vegetables and 9 fruits (20). A great number of health beneficial properties have been described for QC, including antioxidant, anti-inflammatory, anti-allergic, anti-obesity, anti-diabetic, antiviral and anticancer activities, as well as the function to ease some cardiovascular diseases (i.e., heart disease, hypertension and high blood cholesterol) and even mood disorders. There are many current articles and reviews in literature about the protective and/or therapeutic effects together with the mechanisms of QC and QC derivatives/metabolites (19, 21–25). Due to its potential health benefits for humans, QC has come into the focus of utilization as a nutraceutical ingredient in food and pharmaceutical industries. In the USA and Europe, QC supplements are commercially purchasable that the Dietary Supplement Label Database (DSLD) lists over 1200 products currently marketed in the USA (26). On the other hand, the possibility of two toxic effects of QC has been reported. One is most likely due to the formation of possible toxic products such as o-quinone during its reactive oxygen species (ROS) scavenging activities; the quinonic oxidation product of QC may easily react with protein thiols, thereby damaging the function of several critical enzymes. Secondly, QC has been reported to display genotoxic effects in vitro while most in vivo animal studies certify that QC is not carcinogenic (21, 22, 25). Contemporary trends for rational waste valorization dictate the implementation of extraction methodologies within the frame of eco-friendly processes. Such a concept would incorporate strategies towards high efficiency, with lower operating costs and energy consumption without generation of further waste streams. Serving this purpose, utilization of specific solid phase extraction (SPE) sorbents prepared for QC recovery from complex plant materials by molecularly imprinting techniques (27–34) or green extraction methods (10, 11, 15) could be useful. Figure 1 Open in new tabDownload slide Chromatograms of red onion peel extract in MISPE application (detection wavelength: 290 nm) (1 hydroxybenzoic acid derivative; 2 CA; 3 QC derivative (glucoside); 4 QC; 5 KM). (A) Chromatograms obtained before (a) and after (b) sample loading into the cartridge. (B) Chromatograms of washing solutions: (a) Washing 1 (5 mL ACN); (b) Washing 3 (4 mL H2O); (c) Washing 5 {4 mL ACN:H2O (50:50, v/v)}. (C) Chromatograms of eluates; (a) Elution 1 and (b)Elution 2. Figure 1 Open in new tabDownload slide Chromatograms of red onion peel extract in MISPE application (detection wavelength: 290 nm) (1 hydroxybenzoic acid derivative; 2 CA; 3 QC derivative (glucoside); 4 QC; 5 KM). (A) Chromatograms obtained before (a) and after (b) sample loading into the cartridge. (B) Chromatograms of washing solutions: (a) Washing 1 (5 mL ACN); (b) Washing 3 (4 mL H2O); (c) Washing 5 {4 mL ACN:H2O (50:50, v/v)}. (C) Chromatograms of eluates; (a) Elution 1 and (b)Elution 2. Molecularly imprinted polymers (MIPs) are specially designed synthetic sorbents with artificially produced recognition sites capable to identify a specific molecule (template or target molecule) in preference to other structurally related compounds (35, 36). The admission of imprinted polymers as SPE sorbents has led this practice to become known as molecularly imprinted SPE (MISPE). The MISPE method has been successfully applied to the solution of several challenging issues in food, biological and environmental analyses and has also been used for selective separation or determination of natural products in complicated samples with high specificity and selectivity of the molecular recognition mechanism (35–37). Due to the mentioned health beneficial properties, QC is an attractive template molecule, as confirmed by the significant number of papers reporting the manufacture of QC-imprinted polymers (QC-MIPs) for recovery from onion, onion wastes and other plant-derived sources (27–34). In this work, we aimed to convert the waste (onion peel) to a valuable product (QC) using QC-MIP prepared in a simple way (28) as SPE sorbent. Red onion peel was preferred, because many studies (with some exceptions) stated that the richest part in QC and its glycosides of onion is skin (or peel) (2, 13–18) considered as waste, and the red ones are generally richer than yellow and white ones (15–17). As a consequence, we report the successful recovery of QC from red onion peel methanolic extract and its hydrolyzate by using the prepared MISPE sorbent with high yields. Experimental Chemicals The following chemicals were supplied from the indicated sources: methanol (MeOH), acetonitrile (ACN), sodium sulfate and acetic acid (HAc) (Sigma-Aldrich, Steinheim, Germany); catechin (CAT) hydrate, chlorogenic acid (CA) and kaempferol (KM) (Sigma, Steinheim, Germany); QC hydrate, 4-vinylpyridine (4-VP), ethylene glycol dimethacrylate (EDMA) and 2,2′-azobisisobutyronitrile (AIBN) (Aldrich, Steinheim, Germany); o-phosphoric acid, acetone (AC), ethanol (EtOH), HCl 37% (w/v) and dimethyl sulfoxide (DMSO) (Merck, Darmstadt, Germany). Phenolic compound standards were of the purest form. All other chemicals and solvents used were of analytical and high-performance liquid chromatography (HPLC) grades, respectively. Before synthesis, AIBN was recrystallized from MeOH at 45°C. Instruments The extraction and dissolution operations were made using a Bandelin Sonorex (Berlin, Germany) ultrasonic bath. The Soxhlet extractions were carried out with a Heidolph magnetic stirrer (Schwabach, Germany) with heating. The synthesis and hydrolysis operations were made using a GFL 1008 (Burgwedel, Germany) and a HB4 basic IKA-Werke (Staufen, Germany) water bath. The evaporation of the extracting solvent was performed in a Büchi R210/215 (Flawil, Switzerland) rotary evaporator. The SPE processes were carried out using an Agilent (Waldbronn, Germany) 12-port SPE system, ISOLAB (Istanbul, Turkey) vacuum pump and Hamilton (Bonaduz, Switzerland) 1 and 3 mL SPE cartridges. The sieving of the polymers to 100–150-μm particle sizes was performed with a Retsch Vibratory Sieve Shaker (Haan, Germany). The agitation of solutions was made with an Elektro-Mag vortex (Istanbul, Turkey). The double-distilled water was obtained from the Millipore Synergy 185 (France) double-distilled water system. The absorbance measurements and spectral recording were made with a Shimadzu 2600 (Kyoto, Japan) UV-Vis spectrophotometer equipped with a pair of matched Hellma (Müllheim, Germany) quartz cuvettes. The HPLC separations were performed with a Waters Breeze 2 Model HPLC equipment (Milford, MA, USA) consisting of a 2998 photodiode array (PDA) detector and a binary 1525 gradient pump. HPLC data analyses were made with Empower 2 software (Waters Associates, Milford, MA, USA). HPLC analysis The analyses were carried out using an ACE (Aberdeen, Scotland, UK) C18 analytical column (4.6 mm × 250 mm, 5 μm). A new binary gradient elution program was developed and applied with two different solutions: Solvent A (MeOH) and Solvent B (0.2% (v/v) o-H3PO4 in double-distilled water). A total time of 22 min was allowed for the gradient elution program, which was applied as follows: 3 min 5% A, 2 min from 5 to 35% A, 11 min from 35 to 80% A and 6 min from 80 to 100% A (a gradient with slope of 2.0 was applied for each step; using Empower 2 Software, Waters Corporation). The flow rate was 1 mL min−1; analytical detection wavelengths were selected as 290 nm to observe all phenolic constituents and 360 nm to quantify QC and QC derivatives. Using the above working modes, the calibration curves and linear equations of peak area vs. concentration were determined for QC and KM with duplicate injections of 25 μL aliquots of standard solutions at five different concentrations. Preparation of QC-imprinted polymer The preparation and optimization parameters of QC-MIPs were investigated in our previous work (33). Therefore, QC-MIP was thermally prepared by a non-covalent approach through bulk polymerization in 1:4:20 T (template):M (functional monomer):CrL (cross-linking monomer) molar ratio by using 4-VP as the functional monomer, EDMA as the cross-linker and AIBN as the initiator in the porogen of AC. The synthetic procedure for preparation of the 1:4:20 QC:4-VP:EDMA polymer was as follows: 0.798 mmol (0.27 g) QC and 3.192 mmol (0.35 mL) 4-VP were dissolved in AC (15 mL) in a glass vial. This mixture was agitated for 10 min for pre-polymerization. Then, 15.96 mmol (3 mL) EDMA and 0.304 mmol (50 mg) AIBN were added into this mixture. The mixture was placed in an ice-bath and purged with nitrogen gas for 15 min to remove oxygen and establish an inert atmosphere. Afterwards, the glass vial was sealed, placed in a water bath at 60°C and left to stand for 24 h for the polymerization to proceed. The glass vial was smashed for the removal of the polymer. Thereafter, the grounded polymer was washed with methanol:acetic acid (MeOH:HAc) (4:1, v/v) (250 mL × 2) and MeOH (250 mL × 2) in a Soxhlet extraction system and ACN in a shaking water bath sequentially in order to remove the template and remaining unreacted monomers and soluble oligomers until a stable baseline in the UV spectrum of washing solvent was obtained. The collected polymers were sieved to particles of 100–150 μm size. The polymers were dried in a vacuum oven at 50°C. The same procedure was applied for the preparation of non-MIP {NIP (0:4:20)} except for the addition of template in the synthesis step. Preparation of red onion peel extract Red onions purchased from a local market were peeled. A mass of 18 g of peels (dry scales) shredded into small pieces was extracted with aqueous 70% (v/v) MeOH to prepare red onion peel extract in stoppered flasks placed in an ultrasonic bath. The extraction was performed with the following solvent volumes and extraction times in three steps: 50 mL for 60 min, 30 mL for 45 min and 20 mL for 15 min, respectively. In the second and third steps, fresh solvent fractions were added after removal of the former solvent. At the end, all solvent fractions (extracts) were combined and completed to a final volume of 100 mL. A volume of 45 mL of the extract was evaporated under vacuum at 40°C. Then, the residue was dissolved with 25 mL of ACN:DMSO (98:2, v/v) and dried with anhydrous Na2SO4. The red onion peel extract filtered through a filter paper was kept at −18°C to prevent its degradation until MISPE experiments. Preparation of hydrolyzed red onion peel extract Fifty milliliters of the extract prepared as described in the previous section was diluted to 70 mL with water containing HCl so that the final MeOH and HCl concentrations are 50% (v/v) and 1.2 mol L−1, respectively. The prepared acidic solution was hydrolyzed at 80°C for 2 hours by heating under reflux (20). Because acid and water would decrease the adsorption onto the polymer, 20 mL of the red onion peel hydrolyzate was evaporated under vacuum at 40°C. Then, the residue was dissolved with 8 mL of ACN:DMSO (98:2, v/v) and dried with anhydrous Na2SO4. The hydrolyzed red onion peel extract filtered through a filter paper was kept at −18°C to prevent degradation until MISPE experiments. Figure 2 Open in new tabDownload slide Chemical structures of 3,4-dihydroxybenzoic acid (1), CA (2), QC-4′-O-β-glucoside (3), QC (4) and KM (5). Figure 2 Open in new tabDownload slide Chemical structures of 3,4-dihydroxybenzoic acid (1), CA (2), QC-4′-O-β-glucoside (3), QC (4) and KM (5). Molecularly imprinted SPE applications A mass of 100 mg of QC-MIP suspended in ACN was packed into 3 mL of empty SPE cartridge using a wet packing method, and a polytetrafluoroethylene (PTFE) disc frit was fitted on the bottom and the top of the MIP bed. Then, the polymer in the cartridge was conditioned with ACN. QC was isolated from the red onion extract and the hydrolyzed extract using these cartridges. Flow rates of MISPE application steps were optimized as 0.3, 1 and 0.5 mL min−1 for loading, washing and eluting operations, respectively. All eluates diluted at 1:1 (v/v) ratio with water before injection of MISPE steps were evaluated by HPLC chromatograms, and the corresponding QC recoveries were calculated using the linear equation established for QC standard at 360 nm. QC isolation from red onion peel extract by QC-MISPE application A volume of 0.5 mL of the extract was diluted to 2.8 mL with ACN and dried again with anhydrous Na2SO4. Then, 1.3 mL extract was loaded into 100 mg of QC-MIP conditioned with 10 mL ACN in a 3 mL SPE cartridge. Washing operations consisted of six steps {firstly with 5 mL ACN, then two times with 4 mL H2O (totally 8 mL), and three times with ACN:H2O (50:50, v/v) (6, 4 and finally 2 mL, totally 12 mL)}. The elution was carried out with MeOH:HAc (4:1, v/v) for two successive times consisting of 6 and 3 mL, respectively. QC isolation from hydrolyzate of red onion peel extract by QC-MISPE application A volume of 0.5 mL of the hydrolyzate of extract was diluted to 5 mL with ACN and dried again with anhydrous Na2SO4. Then, 1.3 mL of the hydrolyzate was loaded into 100 mg of QC-MIP conditioned with 10 mL ACN in the 3 mL SPE cartridge. Washing operations consisted of six steps {ACN (6 mL), H2O (8 mL), ACN:H2O (20:80, v/v) (6 mL), ACN:H2O (40:60, v/v) (4 mL), ACN:H2O (50:50, v/v) (1 mL), ACN (4 mL), respectively}. The elution was carried out with MeOH:HAc (4:1, v/v) for two successive times consisting of a total volume of 8 mL. Figure 3 Open in new tabDownload slide Chromatograms of hydrolyzate of red onion peel extract in MISPE application (detection wavelength: 290 nm) (1 and 2: CAT derivatives; 3: anthocyanidin derivative; 4: QC; 5: KM). (A) Chromatograms obtained before (a) and after (b) sample loading into the cartridge. (B) Chromatograms of washing solutions: (a) Washing 1 (6 mL ACN); (b) Washing 2 (8 mL H2O); (c) Washing 4 {4 mL ACN:H2O (40:60, v/v)}; Washing 5 {1 mL ACN:H2O (50:50, v/v)}. (C) Chromatogram of eluate {8 mL MeOH:HAc (4:1, v/v)}. Figure 3 Open in new tabDownload slide Chromatograms of hydrolyzate of red onion peel extract in MISPE application (detection wavelength: 290 nm) (1 and 2: CAT derivatives; 3: anthocyanidin derivative; 4: QC; 5: KM). (A) Chromatograms obtained before (a) and after (b) sample loading into the cartridge. (B) Chromatograms of washing solutions: (a) Washing 1 (6 mL ACN); (b) Washing 2 (8 mL H2O); (c) Washing 4 {4 mL ACN:H2O (40:60, v/v)}; Washing 5 {1 mL ACN:H2O (50:50, v/v)}. (C) Chromatogram of eluate {8 mL MeOH:HAc (4:1, v/v)}. Results QC isolation from red onion peel extract by QC-MISPE application A volume of 1.3 mL of the extract with diluted ACN was loaded onto the 3 mL SPE cartridge containing 100 mg QC-MIP at a flow rate of 0.3 mL min−1. The chromatograms of all MISPE step solutions are shown in Figure 1. It can be observed from Figure 1A that the red onion peel extract contains hydroxybenzoic acid derivative, CA, QC derivative (glucoside), QC and KM (Figure 2). In the chromatogram, the compounds except CA, QC and KM were identified by their PDA spectra and literature data. The QC contents of the extract and its hydrolyzate, calculated using linear regression equation (1) obtained with QC standard solutions at different concentrations, were 1.14 and 2.47 mg g−1 dry weight (DW) in our study. $$\begin{equation} y = 7.9 \times 10^{9} c - 3.6 \times 10^{4} \qquad r = 0.9996 \end{equation}$$ (1) (y = peak area; c = molar concentration; r = correlation coefficient) Table I QC-MISPE Results for the Red Onion Peel Extract and the Extract Hydrolyzate Sample Amount of QC (mmol) Amount of KM (mmol) Red onion peel extract Loaded 2.85 × 10−4 3.85 × 10−5 Adsorbed 2.80 × 10−4 3.85 × 10−5 Recovered 1.66 × 10−4 (58%) 2.15 × 10−5 (56%) Hydrolyzate of red onion peel extract Loaded 3.41 × 10−4 3.41 × 10−5 Adsorbed 3.32 × 10−4 3.32 × 10−5 Recovered 2.84 × 10−4 (86%) 2.73 × 10−5 (82%) Sample Amount of QC (mmol) Amount of KM (mmol) Red onion peel extract Loaded 2.85 × 10−4 3.85 × 10−5 Adsorbed 2.80 × 10−4 3.85 × 10−5 Recovered 1.66 × 10−4 (58%) 2.15 × 10−5 (56%) Hydrolyzate of red onion peel extract Loaded 3.41 × 10−4 3.41 × 10−5 Adsorbed 3.32 × 10−4 3.32 × 10−5 Recovered 2.84 × 10−4 (86%) 2.73 × 10−5 (82%) Open in new tab Table I QC-MISPE Results for the Red Onion Peel Extract and the Extract Hydrolyzate Sample Amount of QC (mmol) Amount of KM (mmol) Red onion peel extract Loaded 2.85 × 10−4 3.85 × 10−5 Adsorbed 2.80 × 10−4 3.85 × 10−5 Recovered 1.66 × 10−4 (58%) 2.15 × 10−5 (56%) Hydrolyzate of red onion peel extract Loaded 3.41 × 10−4 3.41 × 10−5 Adsorbed 3.32 × 10−4 3.32 × 10−5 Recovered 2.84 × 10−4 (86%) 2.73 × 10−5 (82%) Sample Amount of QC (mmol) Amount of KM (mmol) Red onion peel extract Loaded 2.85 × 10−4 3.85 × 10−5 Adsorbed 2.80 × 10−4 3.85 × 10−5 Recovered 1.66 × 10−4 (58%) 2.15 × 10−5 (56%) Hydrolyzate of red onion peel extract Loaded 3.41 × 10−4 3.41 × 10−5 Adsorbed 3.32 × 10−4 3.32 × 10−5 Recovered 2.84 × 10−4 (86%) 2.73 × 10−5 (82%) Open in new tab As it can also be seen from these chromatograms, QC and KM (with the absence of 3′-OH in the QC structure) showed strong affinity to QC-MIP, whereas other analogic compounds as well as the structurally related and unrelated compounds in the sample were adsorbed onto the QC-MIP with weak interactions. This situation was expected, because these phenolic compounds have the ability to form hydrogen bonds by their OH groups without entering into the cavities of the MIP. Also the QC derivative (glucoside), probably due to steric hindrance, cannot reach the tailor-made binding sites of MIP. In addition, they can have non-specific ionic interactions with the heterogeneous binding sites of the MIP. Taking advantage of this property, these compounds, except QC and KM, were removed by the washing process consisting of six steps (Figure 1B) so as to overcome weaker interactions with the MIP. Consequently, the elution was performed with 6 and 3 mL of MeOH:HAc (4:1, v/v) as two steps at a flow rate of 0.5 mL min−1, respectively (Figure 1C). It can be observed from these chromatograms that other compounds, except QC and KM, were completely removed practically, and QC was obtained with 58% yield from the red onion extract in two elution steps (Table I). KM amount is calculated using linear regression equation (2) obtained with KM standard solutions at different concentrations. Because KM belongs to the same flavonoid class (flavonol) with QC, its chemical structure is very similar to that of QC, so it was co-eluted with 56% yield (at approximately 1:8 ratio of QC). $$\begin{equation} y = 7.2\times10^{9} c - 1.8 \times 10^{4} \qquad r = 0.9996 \end{equation}$$ (2) QC isolation from hydrolyzate of red onion peel extract by QC-MISPE application It is obvious that conversion of the QC derivative (glucoside) to QC will increase the QC amount isolated from onion peel; therefore, QC-MISPE was also applied to the hydrolyzate of the extract. A volume of 1.3 mL of the hydrolyzate with diluted ACN was loaded onto the 3 mL SPE cartridge containing 100 mg QC-MIP at a flow rate of 0.3 mL min−1. The chromatograms of all MISPE step solutions are shown in Figure 3. It can be observed from Figure 3A that the hydrolyzate of red onion extract contains CAT derivatives, anthocyanidin derivative, QC and KM. In the chromatogram, components other than QC and KM were identified by their PDA spectra and literature data. The hydrolyzate did not contain the QC derivative as expected. As can be seen from these chromatograms, other compounds except QC and KM were almost completely removed by the washing process, and QC was obtained with 86% yield from the hydrolyzate of red onion peel extract with the elution step (Table I). KM was also co-eluted with 82% yield (at approximately 1:8 ratio of QC). Due to the fact that KM is also a health beneficial flavonoid (38), it was decided not to increase the washing steps for complete removal of KM because this would also decrease the QC recovery yield. As shown, the matrix compounds in the red onion peel extract did not complement the imprinting cavities of the MIP, leading to weak interactions. Discussion As in many studies, the main constituents of red onion peels (Figure 1A) were QC, QC derivative (glucoside) and KM in the flavonoid class (6, 11, 15, 18), and CA in the hydroxycinnamic acid class of polyphenolic compounds. Hydroxycinnamic acid derivatives identified in onion waste highly vary in literature. Ferulic acid and protocatechuic acid were found in one study (6), while in another, protocatechuic acid and its derivative were identified (10). In our study, the QC contents of the extract and its hydrolysate (1.14 and 2.47 mg g−1 DW, respectively) may seem slightly low; it can be accepted as compatible with literature values because there are many factors such as varieties, environmental and agronomic growth conditions, cultivar and storage time influencing QC amounts in peels (2, 5). The amounts of 2.99 mg g−1 QC and 1.15 mg g−1 KM (18), 3.9 mg g−1 QC (after hydrolysis) and 0.23 mg g−1 KM (6) and 52 mg g−1 (in extract) and 62 mg g−1 QC (after enzymatic hydrolysis of the extract) (15) were found in various studies with red onion peel. In another study (11), the QC and KM contents were found as 17.63 and 0.88 mg g−1 in orange-colored outer skin of onion. These components were identified in yellow onion skin as 1.78 and 0.26 mg g−1, respectively (8) and 1.61 mg g−1 (QC) (2). QC-MISPE was also applied to the hydrolyzate of the extract because the hydrolysis process was thought to increase the amount of QC isolated from the onion peel. In support of this, Choi et al. observed that, after enzymatic hydrolysis with cellulase, pectinase and xylanase enzyme mixture, the QC recovery (75%) in the onion skin waste extract reached a value as high as 1.61-fold, which was higher than that obtained by extraction from an untreated sample with enzyme mixture in the study aiming to utilize onion skin waste as a valorization resource for sugar and QC (3). In another study, Turner et al. developed an environmentally sustainable procedure using pressurized hot water to extract the QC species with a higher recovery yield, followed by biocatalytic conversion of the QC glycosides to QC and carbohydrates (15). In the chromatogram obtained after hydrolysis (Figure 3A), QC glycosides are not as high as expected. The CAT derivatives determined in the hydrolyzate chromatogram are probably due to anthocyanidins during the hydrolysis process, because CATs are intermediates in the biosynthesis of anthocyanidins (39). The increase in recovery yield from the hydrolyzate of the extract could be explained by the weaker interactions of CAT derivatives in the hydrolyzate than CA in the extract with QC-MIP. In other words, CAT in the hydrolyzate could be washed away more easily than CA in the main extract, due to structural properties. In a previous study, we found CAT as the second lowest adsorbed polyphenolic after vanillic acid (28). For this reason, it was possible to remove the main matrix compounds in the hydrolyzate easily and to recover QC with a higher yield. In earlier studies, Ko et al. extracted QC from onion skin using subcritical water with 92.4% recovery yield (11), and Choi et al. recovered with 75% yield (3). Especially the latter study could be criticized for its laboriousness because both enzymatic hydrolysis and separation steps with the developed nano-matrix (terpyridine-immobilized silica-coated magnetic nanoparticles-zinc(II)) were used. Hassan et al. demonstrated QC recovery from onion solid waste as 260 mg kg−1 onion peel using QC-MIP (1:10:100 molar ratio QC:methacrylic acid (MAA):EDMA) (30). This value is lower than those reported in our study (670 and 2054 mg kg−1 onion peel from methanolic extract and its hydrolysate, respectively). In addition, in the same study, the eluate contains QC-7,4′-diglucoside, isorhamnetin 3,4′-diglucoside and isorhamnetin 4′-glucoside together with QC. For this reason, QC could not be recovered selectively. Prospects of the proposed technology in selective recovery and scale-up of the target compound may be envisaged for further research on this subject. The target behind optimization of molecular imprinting protocols is to aid in the generation of recognition sites that show a high selectivity and strong binding ability toward the analyte (or recovered compound) while keeping non-selective interactions at a minimal level (40). In multi-step optimization of extractive recovery protocols that has to precede scale-up and operation at a greater level, thermodynamic equilibrium parameters, such as the choices of fixed-bed and mobile phases and eluting solvents that together determine selectivity and efficiency of recovery, can be separated from kinetic parameters (e.g., flow rate, feed loading and particle size) (41). In MISPE, thermodynamic selectivity can be adjusted with the correct choice of molecularly imprinted sorbent and solvent polarity. For example, ergosterol-MIP binding was shown to be enhanced with aqueous organic solvent loading conditions (42); thus, double-fold selectivity can be achieved in MISPE recoveries by selecting the suitable MIP sorbent and the eluting solvent composition (such as partially aqueous solvent mixtures). Another example is the MISPE recovery of caffeic acid phenethyl ester (CAPE) and caffeic acid (CA)—having a close structure—from populous buds with MISPE, which yielded better results than C18-SPE, so both compounds could be isolated and enriched simultaneously (43). In conventional scale-up of MISPE (which is a matter of more advanced research and experimentation), once the optimal fixed-bed sorbent is selected, further parameters are searched for best analyte loading and pH, the flow rates for feed and elution, gradient slope and column length. During scale-up studies, equivalence should be preserved between the laboratory and pilot plant chromatographic columns in terms of kinetic (particle and pore sizes, eluting solvent, temperature, etc.) and dynamic (bed height, flow rate, pack density, etc.) factors. Other factors being maintained at the same level, the increased volume of feed load at pilot scale should be handled by increasing the column diameter while keeping the bed height constant, so as to obtain a proportional volume increase and a constant residence time of the target product (41). Conclusion In this study, the presence of polyphenolic components (QC and QC glucoside as flavonoids and CA as hydroxycinnamic acid) of red onion skin were confirmed, similar to the findings of literature studies. The amounts of active components differed in a wide range according to varieties, environmental and agronomic growth conditions, cultivar and storage time. For red onion peels, QC contents were in the 2.99–83.48 mg g−1 DW range and for yellow ones 1.57–34.43 mg g−1 DW. The amount of QC was found in our study as 1.14 (in extract) and 2.47 mg g−1 DW (in hydrolyzate). Although this value is slightly lower, it can be accepted as compatible literature values. The recovery yields of QC from the extract and its hydrolyzate were found as 58 and 86%, respectively, using the developed MISPE method. It was observed that the hydrolysis process increased the QC amount as expected and also increased the overall recovery yield due to changing matrix components, which could easily be washed away without significant QC loss. As onion peel is a food processing waste (actually posing an economic and environmental problem due to its huge amounts) containing a valuable product (QC) known to be marketed in USA among more than 1200 commercial products listed by the DSLD, this work has achieved the recovery of QC with a suitable material (QC-MIP) and process (MISPE with optimum loading, washing and eluting conditions) in a simple way. It was also possible to increase the QC yield (from 58 to 86%) significantly by hydrolysis. 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TI - Valorization of Red Onion Peels for Quercetin Recovery Using Quercetin-Imprinted Polymer
JO - Journal of Chromatographic Science
DO - 10.1093/chromsci/bmz079
DA - 2020-01-23
UR - https://www.deepdyve.com/lp/oxford-university-press/valorization-of-red-onion-peels-for-quercetin-recovery-using-quercetin-0Tadh06IcT
SP - 1
VL - Advance Article
IS - 
DP - DeepDyve
ER -